A review of SLIT studies reveals that both cumulative dose and maintenance schedules have widely varied between the published clinical trials. Pre-co-seasonal treatment for pollen allergy seems to be better than coseasonal treatment. There is a number of studies evaluating different durations of SLIT, but the heterogeneity of the study design, allergen extracts and outcome measures make it difficult to produce a meta-analysis on SLIT efficacy according to duration. Overall,duration of both the build-up phase and the maintenance treatment does not seem to influence the effectiveness of SLIT. This suggests that the quality of the extract is more important in determining the clinical response to SLIT than duration of therapy. Further studies are warranted to establish optimal duration of SLIT treatment.